There could be several reasons why your SIM card is not working on your iPad. Here are a few things you can check:
Make sure your iPad is compatible with the type of SIM card you have. Some iPads only work with specific types of SIM cards, so it's important to check if your device is compatible.
Check if the SIM card is inserted correctly. Make sure the SIM card is inserted properly in the SIM card tray. If it's not inserted correctly, your iPad may not recognize it.
Check if your iPad is unlocked. If your iPad is carrier-locked, it may not work with a different carrier's SIM card. Contact your carrier to confirm if your iPad is unlocked or not.
Check if your iPad has cellular connectivity. Make sure your iPad is a cellular model and not a Wi-Fi-only model. If it's a Wi-Fi-only model, it won't work with a SIM card.
Check if the SIM card is activated. If your SIM card is not activated, your iPad won't be able to use it. Contact your carrier to activate your SIM card.
If none of these solutions work, contact your carrier or Apple support for further assistance